This invention relates to a method for reducing a high molecular weight polydimethylglutarimide polymer to a lower molecular weight polymer, to obtaining polymers having a low polydispersity, and the use of these polymers in a positive photoresist system.
Photoresist compositions are often used as masks to protect substrates from chemical etchants in photoengraving processes. This property has resulted in their use in the manufacture of printed circuit boards and semiconductors. For use in the manufacture of a semiconductor, the photoresist must possess chemical resistance, must adhere to the circuit substrate, and for high density circuits, must be capable of fine line resolution.
Positive photoresists comprise a light sensitive compound in a film-forming polymeric resin binder. The resins improve the adhesion of the coating composition in these photoresists. They further fortify the coat for better chemical resistance, reduce the tendency for sensitizers to crystallize out, increase the viscosity encoding for coating characteristics and reduce the costs of the resist solution as a partial substitute for sensitizers.
Polyimide resins, in particular, have a number of properties which make them extremely useful when incorporated in positive photoresists because they are very sensitive to both electronic beam and light irradiation. Further, they improve the thermal stability of such photoresists.
U.S. Pat. No. 3,964,908, incorporated herein by reference, describes a process for forming an image using a polydimethylglutarimide polymer having high thermal stability. This polymer is formed directly on the photoresist itself. Consequently, the photoresist system formed cannot contain a sensitizer because known photosensitizers thermally decompose at temperatures below that used in the in-situ polymerization reaction used to form this resist.
U.S. Pat. No. 4,524,121 to Rohm and Haas, which is incorporated herein by reference, describes a photoresist system that contains a "preformed" polyglutarimide polymer. By using a "preformed" polymer, the photoresist formed can include a sensitizer. Accordingly, the resulting photoresist has higher thermal stability and good image resolution without the need for subsequent film hardening while possessing excellent transparency over a wider range of exposure frequencies than typical phenolic (Novolak) coupler resins. A process for forming polyglutarimide polymers is disclosed in U.S. Pat. No. 4,246,374. The molecular weight of the polyglutarimide resin used by U.S. Pat. No. 4,5224,121, is stated to be preferably at least about 50,000 weight average molecular weight in contrast to the 10,000 average molecular weight of phenolic resins. This high a molecular weight for the resin can cause processing problems when the polymer is incorporated into certain resist products. The viscosity of resultant solutions is excessive when solutions of sufficiently high solid contents are formulated to provide films of the proper thickness. The thin films obtained are often very badly striated.
It is difficult under normal commercial manufacturing processes to prepare polyglutarimide polymers with a wide range of molecular weights. Typically, the resin produced has a molecular weight in excess of 100,000 weight average molecular weight, more typically, the weight is about 140,000 weight average molecular weight. A simple process that would enable users to lower the molecular weight of these high molecular weight polydimethylglutarimide polymers and thereby decrease the solution viscosity of the polymer to meet the particular demands of specific applications would be useful. Such a process would be particularly useful in preparing various coatable resist compositions, such as photoresists, X-ray resists and electron beam resists. It is also important that the process used in lowering the molecular weight of the high molecular weight polymers result in relatively uniform molecular weight polymers.
A process for reducing the molecular weight of high molecular weight polydimethylglutarimide has now been discovered. This process also results in polymers having the low polydispersity, i.e., a narrow range of molecular weights. Additionally, this process can be used in preparing a high quality, aqueous base developable positive resist film on a substrate for lithographic applications.